Plant Genome III Conference

AN APPROACH FOR CLASSIFYING FUSIFORM RUST DISEASE RESISTANCE: GENOMIC MAPPING AND CATALOGING OF RESISTANCE GENES.

1. Department of Forestry, N.C.State University, Raleigh, NC 27695
2. USDA-FS, Forestry Sciences Lab, Athens, GA 30602

Fusiform rust (FR), a gall forming disease of southern pines, causes annual losses of 36 to 192 million dollars, and is widely viewed as the leading source of disease loss in southern forests. While pine breeders have improved resistance levels in planting stocks, the genetic basis of FR disease resistance is poorly understood. Previously, most genetics and breeding efforts approached FR resistance as a quantitative (polygenic) trait, involving may small effects genes. Our research group in 1992 proposed genetic dissection of FR resistance in loblolly pine, through detection of RAPD marker-phenotype associations. With Plant Genome support, we identified and mapped a single major effect (qualitative) gene (Fr1), controlling gall presence or absence in clone 10-5. In 10-5 progeny, Fr1 was shown to confer significant resistance in both greenhouse and filed studies. Existing rust phenotypic data suggest other major genes for FR resistance occur in several loblolly families. We hypothesize that many major genes could be involved in durable resistance in forest trees. Now we can test this idea for FR resistance by genetic mapping and interaction cataloging. Here, we present an approach using genetic markers to identify additional FR resistance genes, and in conjunction with virulence characterized fugal isolates catalog host-pathogen (gene-isolate) interactions for use in a resistance classification scheme. In this approach, phenotypic data will be used to select families that appear heterozygous for resistance (gall absence vs. presence). Ideally, full-sib families with a resistant and a susceptible parent would be selected, but open pollinated families can be used. Families will be challenged with inocula from an array of single aeciospore isolates (SAIs). DNA from the families (diploid from seedlings or haploid from magagametophytes) will be used to develop RAPD markers. Via bulk segregant analysis, cosegregation analysis and/or genomic mapping we will seek significant marker-resistant trait associations, which are indicative of resistance gene/avirulence gene interactions. Resistance genes so identified (marker tagged) will be differentiated from each other by map location through syntenic markers and anchorage with known single copy genes. Each resistance gene (family)-SAI interaction will be cataloged and used to comparatively classify resistance in unknown germplasm. Where 2 or more known resistance gene (family)-SAI combinations show reciprocal interactions, responses differing from those seen with the known families may still be used to infer or predict a genotype for the unknown.